In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that arc most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendaring rollers at the end of the process. Papermaking devices are well known in the art and are described, for example, in “Handbook for Pulp & Paper Technologists” 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and “Pulp and Paper Manufacture” Vol 111 (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat Nos. 5,539,634 to He, 5,022,966 to Hu, 4,982,334 to Balakrisbnan, 4,786,817 to Boissevain et al, and 4,767,935 to Anderson et at. Process control techniques for papermaking machines are further described, for instance, in U.S. Pat Nos. 6,149,770 to Hu et al., 6,092,003 to Hagart-Alexander et. al, 6,080,278 to Heaven et al., 6,059,931 to Hu et al., 5,853,543 to Hu et al., and 5,892,679 to He.
On-line measurements of sheet properties can be made in both the machine direction and in the cross direction. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
Papermaking machines typically have several control stages with numerous, independently-controllable actuators that extend across the width of the sheet at each control stage. For example, a papermaking machine will typically include a headbox having a plurality of slice lip force actuators at the front which allow the stock in the headbox to flow out on the fabric of the web or wire. The papermaking machine might also include a steam box having numerous steam actuators that control the amount of heat applied to several zones across the sheet. Similarly, in a calendaring stage, a segmented calendaring roller can have several actuators for controlling the nip pressure applied between the rollers at various zones across the sheet.
All of the actuators in a stage are operated to maintain a uniform and high quality finished product. Such control might be performed, for instance, by an operator who periodically monitors sensor readings and then manually adjusts each of the actuators until the desired output readings are produced. Papermaking machines can further include computer control systems for automatically adjusting cross-directional actuators using signals sent from scanning sensors.
In making paper, virtually all MD variations can be traced back to high-frequency or low-frequency pulsations in the headbox approach system. CD variations are more complex. Preferably, the cross-directional dry weight profile of the final paper product is flat, that is, the product exhibits no CD variation, however, this is seldom the case. Various factors contribute to the non-uniform CD profiles such as non-uniformities in pulp stock distribution, drainage, drying and mechanical forces on the sheet. The causes of these factors include, for example, (i) non-uniform headbox delivery, (ii) clogging of the plastic mesh fabric of the wire, (iii) varying amounts of tension on the wire, (iv) uneven vacuum distribution, (v) uneven press or calendar nip pressures, and (vi) uneven temperatures and airflows across the CD that lead to moisture non-uniformities.
Cross-directional measurements are typically made with a scanning sensor that periodically traverses back and forth across the width of the sheet material. The objective of scanning across the sheet is to measure the variability of the sheet in both CD and MD. Based on the measurements, corrections to the process are commanded by the control computer and executed by the actuators to make the sheet more uniform.
In practice, control devices that are associated with sheetmaking machines normally include a series of actuator systems arranged in the cross direction. For example, in a typical headbox, the control device is a flexible member or slice lip that extends laterally across a small gap at the bottom discharge port of the headbox. The slice lip is movable for adjusting the area of the gap and, hence, for adjusting the rate at which feedstock is discharged from the headbox. A typical slice lip is operated by a number of actuator systems, or cells, that operate to cause localized bending of the slice lip at spaced apart locations in the cross-direction. The localized bending of the slice lip member, in turn, determines the width of the feed gap at the various slice locations across the web.
It is standard practice that sheetmaking machines be controlled by adjusting actuators using measurement signals provided by scanning sensors. In the case of cross-directional control, for example, a commonly suggested control scheme is to measure values at selected cross direction locations on a sheet and then to compare those measured values to target or set point values. The difference for each pair of measured and set point values, i.e., the error, can be used for algorithmically generating appropriate outputs to cross direction control actuators to minimize the error. In such systems, a measurement zone is defined as the cross direction portion of sheet which is measured and used as feedback control for a cross direction actuator zone, and a control zone is defined as the portion of the sheet affected by a cross direction actuator zone.
In practice, it is difficult to control sheetmaking machines by adjusting actuators using measurement signals provided by scanning sensors. The difficulties particularly arise because the scanning sensors are separated from the control actuators by substantial distances in the machine direction. Because of such separations, it is difficult to determine which measurements zones are associated with which actuator zones. Such difficulties are referred to as alignment problems in the papermaking art. Alignment problems are exacerbated when, as is typical, there is uneven paper shrinkage of a paper web as it progresses through a papermaking process. Another difficulty is that the effect of each actuator is not always limited within the corresponding control zone but spans over a few control zones. Alignment is an important process model parameter for keeping the CD control system stable and operating. The alignment can change over time and subsequently degrade the controller performance and thus paper quality.
One conventional method for aligning actuator zones with measurement zones involves the use of dye tests. In a dye test, narrow streams of colored liquid are applied to feedstock as it flows beneath a slice lip. The dye streams initially form parallel lines that extend in the machine direction, but those lines may deviate from parallel if there is web shrinkage during the papermaking process. The dye marks passing through the measurement devices reveal the distribution of control zones and therefore specify the alignment of measurement zones.
Conventional dye tests, however, have numerous drawbacks. The most serious drawback is that the tests destroy finished product and, therefore, it is seldom feasible to perform dye tests at an intermediate point in a sheetmaking production run, even though sheetmaking processes are likely to drift out of control during such times. Further, because of the limited thickness and high absorption characteristics of tissue grades of paper, dye tests are typically limited to paper products that have relatively high weight grades.
More recently, systems that automatically and non-destructively map and align actuator zones to measurements zones in sheetmaking systems have been developed. Some of these systems perform so-called “bump tests” by disturbing selected actuators and detecting their responses, typically with the CD control system in open-loop. The term “bump test” refers to a procedure whereby an operating parameter on the sheetmaking system, such as a papermaking machine, is altered and changes of certain dependent variables resulting therefrom are measured. Prior to initiating any bump test, the papermaking machine is first operated at predetermined baseline conditions. By “baseline conditions” is meant those operating conditions whereby the machine produces paper of acceptable quality. Typically, the baseline conditions will correspond to standard or optimized parameters for papermaking. Given the expense involved in operating the machine, extreme conditions that may produce defective, non-useable paper are to be avoided. In a similar vein, when an operating parameter in the system is modified for the bump test, the change should not be so drastic as to damage the machine or produce defective paper. After the machine has reached steady state or stable operations, the certain operating parameters are measured and recorded. Sufficient number of measurements over a length of time is taken to provide representative data of the responses to the bump test.
The standard bump test for CD model identification includes the following steps: (1) placing a control system in open-loop; (2) bumping a subset of the actuators at the headbox to follow a step or series of steps in time; (3) collecting the output data as measured by sensor(s) in the scanner; and (4) running a model identification algorithm to identify the model parameters including alignment.
For example, U.S. Pat. No. 5,400,258 to He discloses a standard alignment bump test for a papermaking system in which an actuator is moved and its response is read by a scanning sensor and the alignment is identified by the software. U.S. Pat. No. 6,086,237 to Gorinevsky and Heaven discloses a similar technique but with more sophisticated data processing. Specifically, in their bump test the actuators are moved and technique identifies the response as seen by the scanner.
With current bump test alignment methods, the operator can identify the alignment at the time of the bump test experiment. To track alignment changes over time there is a need to re-identify alignment over the course of days and weeks. Moreover, model identification for a system in closed-loop control is well known to be challenging. This is due in part to the fundamental reason that a closed-loop control system works to eliminate any perturbations, so prior art techniques have endeavored to “sneak” a perturbation into the actuator profile that works against the rest of the system and attaining sufficient excitation of the system is difficult to achieve.